Al-Ca master alloy was prepared by molten salt electrolysis method, using a mixed molten salt system of CaCl2-CaF2 and feeding CaO as electrolysis material. The technological parameters such as back electromotive force (BEMF), cell voltage, and current were measured by testing device of molten salt electrolysis. The effects of current density, CaO addition, electrolysis time and feeding period on the back electromotive force in the electrolysis process are studied. The results indicate that back electromotive force increases with current density increasing, the feeding period is 30min measured by potential control method. Finally, the Al-Ca alloy with the Ca content of 11.6wt% can be obtained by electrolyzing for 1h at the current of 7A and at 740.